Spark plugs which have pre-spark gaps to discharge a capacitor and thereby apply the energy stored in the capacitor to the main spark gap have previously been proposed, see, for example, U.S. Ser. No. 649,989, filed Sept. 13, 1984, Herden et al.
Spark plugs which have pre-spark/or pilot spark gaps and capacitors as described are particularly suitable for reliably igniting even very lean fuel-air mixtures which provides for excellent energy transfer from the mixture to the engine, and insures complete combustion of any fuel in the fuel-air mixture. Complete combustion results in low polluting emissions from the engine and high utilization of the fuel being used. Spark plugs of this type have the additional advantage that they are directly replaceable with conventional spark plugs in engines having any type of ignition system. Reliable ignition of even lean fuel-air mixtures is obtained by converting a substantial portion of the available ignition energy during the breakdown phase of the initial phase of the spark itself.
It has been found that the pre-spark gap, which controls discharge of the capacitor within the spark plug, does cause some loss in energy which, most desirably, should be transferred to the main spark gap itself. The pre-spark, thus, reduces the available ignition energy which, in the end, causes burning of the fuel-air mixture within the combustion chamber of the engine.